A new distant giant radio galaxy in the Boötes field serendipitously detected by Chandra

¹ SISSA – International School for Advanced Studies, Via Bonomea 265, 34151 Trieste, Italy
e-mail: amasini@sissa.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁴ INFN – National Institute for Nuclear Physics, Via Valerio 2, 34127 Trieste, Italy
⁵ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁶ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1A, 14000 Praha 4, Czech Republic
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 10 February 2021
Accepted: 9 March 2021

Abstract

Context. Giant radio galaxies (GRGs) are the largest single structures in the Universe. Exhibiting extended radio morphology, their projected sizes range from 0.7 Mpc up to 4.9 Mpc. LOFAR has opened a new window on the discovery and investigation of GRGs and, despite the hundreds that are known today, their main growth catalyst is still under debate.

Aims. One natural explanation for the exceptional size of GRGs is their old age. In this context, hard X-ray selected GRGs show evidence of restarting activity, with the giant radio lobes being mostly disconnected from the nuclear source, if any are present at all. In this paper, we present the serendipitous discovery of a distant (z = 0.629), medium X-ray-selected GRG in the Boötes field.

Methods. High-quality, deep Chandra and LOFAR data allow for a robust study of the connection between the nucleus and the lobes, at a larger redshift, which has thus far been inaccessible to coded-mask hard X-ray instruments.

Results. The radio morphology of the GRG presented in this work does not show evidence for restarted activity and the nuclear radio core spectrum does not appear to be gigahertz-peaked spectrum (GPS)-like. On the other hand, the X-ray properties of the new GRG are perfectly consistent with the ones previously studied with Swift/BAT and INTEGRAL at lower redshift. In particular, the bolometric luminosity measured from the X-ray spectrum is a factor of six larger than the one derived from the radio lobes, although the large uncertainties make them formally consistent at 1σ. Finally, the moderately dense environment around the GRG, traced by the spatial distribution of galaxies, supports recent findings that the growth of GRGs is not primarily driven by underdense environments.